1. Field of the Invention
Present invention relates optical scanner, and particularly that which can satisfy finite and infinite optical systems at same time.
2. Description of the Prior Art
Light modulated beam from laser diode 1 passes slit 3 having long hole form in main scan direction after the beam became almost parallel light beam by first optical element 2 in ordinary optical scanner.
This light beam that has passed slit 3 enters second optical element 4 having refractivity in subscan direction, then this main scan direction light beam that has passed second optical element 4 enters reflection plane 5a of optical deflector 5 as almost a parallel light, then is deflected to third optical element 6 while subscan direction light beam is deflected to third optical element 6 after forming image on reflection plane 5a of optical deflector 5.
Whence if each reflection plane 5a of optical deflector 5 has angular error (wobble) distorted with respect to rotation axis, it may influence bad effect in subscan direction when light beam forms an image on surface 7 where to be scanned.
Japanese Patent Publication Heisei 4-110817 to solve this matter in order to minimize influence from angular error has suggested to use in third optical element a toric surface having different curvature radius for main scan from subscan direction and to constitute an optical conjugate point from the scanned surface and the reflection plane of polygonal rotating mirror.
And U.S. Pat. No. 4,639,072 tried to prevent bad influence in performance owing to angular error (wobble) by way of placing cylinder lens near the scanned surface.
Third optical element images on scanned surface the light beam deflected by optical deflector into elliptic spot having major axis in subscan direction and at the same time satisfies f-xcex8 characteristics for total scan range.
But because phenomenon arose that image formation points in subscan direction appear asymmetrically on right and left of scanned surface because the above described optical conjugate point is not maintained while deflection plane position is changed when optical deflector rotates, U.S. Pat. No. 5,448,502 suggested to minimize shift of imaging point in subscan direction by using for the scanning lens near scanned surface a modified cylinder lens where curvature radii in subscan direction are formed asymmetrically on right and left.
But most of the above described previous techniques are the scanning optical systems that are composed of cases each made of two or three combined pieces of plastic or glass.
Accordingly recently there has been proposed use of third optical element composed in one piece in aspects of cost reduction and equipment compactification.
Third optical element requires to be placed as near as possible to the optical reflector in order not to make itself a large one for good optical performance in broad effective scan range in a compact size of the scanning optical equipment.
But if third optical element is thus placed near optical deflector then magnification of third optical element in subscan direction becomes greater so that position error of spot in subscan direction on scanned surface may become greater in proportion to magnification in subscan direction if the beam come from second optical element as described above is to form a linear image with position error, so that the great position error bring result that image field curvature in subscan direction becomes relatively greater which may bring performance deterioration.
Though it is easy to correct well the image plane in subscan direction in conventional scanning optical equipment having third optical element made of two or three pieces because the equipment has several refraction surfaces in subscan direction, the scanning optical equipment having third optical element made of one piece has particular necessity that image field curvature particularly in subscan direction be corrected well at time of design because the equipment can have only two refraction surfaces in maximum.
But on the contrary in Japanese Patent Publication Heisei 8-76011 and 8-297256, magnification in subscan direction of third optical element has been made somewhat small so that wavefront aberration is increased at the imaging point near scanned surface so that eventually it becomes difficult to make the spot small which is imaged on the whole scanned surface.
Therefore it is important to place well the third optical element between scanned surface and deflection point of optical deflector.
And while polygonal rotating mirror is generally used as deflector, there is problem because rotation axis of polygonal rotating mirror is not on deflectional reflection plane, there is spatial aberration between image formation position and deflectional reflection plane so that there arises asymmetry in right and left with regard to deflection angle of polygonal rotating mirror with reference to scanned surface center point so that the asymmetry eventually causes performance deterioration of third optical element.
Because of such problems it was proposed that it is necessary to use a modified toric at least on one plane in case where third optical element is made of one piece of lens.
U.S. Pat. No. 5,111,219 has proposed to artificially converge beam coming from first optical element and to make third optical element to have meniscus shape having a convex surface in deflection plane.
But though meniscus form proposed in this invention has advantage that it can decrease difference in thickness between center and circumferential part, the form has disadvantage that it may be somewhat sensitive to tolerance because refractivity of each plane becomes large.
Present invention is devised to solve the various problems of such conventional scanning optical equipment so that the purpose is to provide a scanning optical equipment that may attempt optical performance improvement by taking advantages of finite and infinite optical systems together.
The scanning optical equipment according to present invention to achieve such purpose features conformation composed of a laser diode which scans laser beam of definite wavelength; first optical element which patterns the beam scanned from the above laser diode to an almost parallel beam; a slit where long hole has been formed in main scan direction parallel to optical axis which slit is located in front of the above first optical element; second optical element located in front of the above slit, having refractivity in direction perpendicular to optical axis; an optical deflector where reflection plane has been provided which reflects with designated angle the laser beam that has been scanned from the above laser diode and has passed the above first optical element, slit and second optical element; and third optical element whose main scan direction is nonspherical shape and subscan direction is made spherical surface which optical element spots to scanned surface the laser beam reflected from the above optical deflector.
A preferable characteristics of present invention is that the above third optical element satisfies the following condition equations to correct the image field curvature and the distortion aberration characteristics if distance from deflection point to first plane is taken as d, distance from deflection point to scanned surface is taken as L, center thickness of third optical element is taken as S and refractivity of third optical element for used wavelength is taken as n.
(Image field curvature) M1=d/Lxc3x97S, 1.54xe2x89xa6M1xe2x89xa63.28
(Distortion aberration characteristics) M2=M1xc3x97(nxe2x88x921), 0.74xe2x89xa6M2xe2x89xa61.87
Another preferable characteristics of present invention is that the above third optical element is composed of both surfaces that are made of modified toric surface, among which the surface facing the optical deflector has convex shape.
And another preferable characteristics of present invention is to use infinite optical system where light beam that has passed the above first optical element becomes light beam composed of almost parallel components.